It is well known that a conventional interphone system includes a master station inside a house and a remote station installed at an entrance of the house. The stations are electrically connected for a telephonic intercommunication between a visitor and an occupant of the house upon operation of a system actuation switch provided at the remote station. However, this interphone system, has often been unsatisfactory for the house occupant to surely indentify the visitor, because the personal identification is made only through the telephonic or verbal intercommunication.
For elimination of the above problem, there has been an attempt made to annex a video monitor subsystem to a conventional telephonic intercommunication system as shown in FIG. 1 to obtain a more ensured identification of the visitor with the aid of visual information. The conventional video monitor subsystem includes a tube type television camera 4 such as a vidicon installed near the remote station at the entrance for picking up the facial image of the visitor and a video monitor display installed at the master station for displaying the image thereon to provide a helpful visual information for the house occupant. The above system, however, has been problematic for the reason that the television camera 4 tends to give an offensive impression to the visitor with its relatively bulky and conspicuous appearance. It has of course been tried to integrate the vidicon type television camera 4 into the housing of the remote station 1 for alleviation of such an unfavorable factor, which conversely leads to another problem that the vidicon type television camera 4 requires renewal every certain period due to burning of the image plate thereof. The vidicon type television camera 4 integrated into the housing of the remote station brings about the further problem that it necessitates a control circuit provided close thereto controlling the convergence of the electron beam and so forth, which inevitably ends in the increase in size of the housing of the remote station. More precisely as shown in FIG. 2, the vidicon type television camera 4 occupies a considerable amount of the total internal space of the housing 6 of the remote station 1 even with respect to an audio unit U and requires an optical opening of a relatively large size, which again ends in bulky and conspicuous appearance of the remote station 1 giving an unfavorable impression to the visitor.
In addition, the vidicon type television camera 4 being innately designed to be sensitive only to visible daylight is generally operated at night with the assistance of an illumination device to obtain an image of a sufficient brightness, which may induce a far more repugnance of the visitor thereaganist with a feeling of being subjected to severe observation. Such an illumination device 44 has been conventionally mounted or integrated with the television camera 4 within the remote station installed at the entrance as shown in FIG. 2, where the illumination device 44 occupies a greater amount of space to provide light of a sufficient illuminance, which also ends in the dimensional increase of the remote station marring the appearance around the entrance of the house.
Although it is expedient to adopt an image orthicon sensitive to an infrared light for the television camera 4 to overcome the above problem, the adoption of the image orthicon still accompanies the dimensional increase of the remote station as well as the same drawbacks inherent to the vidicon with a practically prohibitive cost for use by the general public.
There is a color camera device sensitive to light of relatively low illuminance as disclosed in the unexamined Japanese Utility Model Publication No. 55-166617. However, such a color camera device is unbenefitting for the interphone of home use because it employs an instrument generally referred to as the image intensifier being which is too large and expensive.
In consideration of the above-mentioned problems, the present invention is made to provide an interphone system with an improved video monitor subsystem.